Five cases of B-cell lymphoma are described in which the morphology and initial immunohistochemistry suggested a diagnosis of T-cell neoplasia. In four cases, the histological picture was that of an adult pleomorphic T-cell lymphoma; the fifth case was a lymphocytic lymphoma (CLL) with an accompanying T-cell lymphocytosis in the peripheral blood. Immunohistochemistry on both frozen and paraffin-embedded material showed that the cellular population in all five cases consisted mainly of T-cells; less than 10% of the cells stained as B-cells. However, immunoglobulin immunostaining combined with use of the new lineage-related monoclonal antibodies reactive in paraffin section revealed that the B-cells constituted the neoplastic population. Genetic analysis showed no evidence of clonality in the T-cells, nor was it able to detect rearrangement in the small number of clonal B-cells present. These cases represent a variety of B-cell neoplasia that mimicks T-cell lymphoma morphologically and immunologically. The vigorous T-cell reaction seen in such lymphomas means that the malignant population can be correctly identified only by careful examination of the immunohistochemical findings.